[jboss-jira] [JBoss JIRA] Created: (JGRP-755) ProtocolStack: verify that TimeScheduler gets garbage collected on redeployment

[Bela Ban (JIRA)]

ProtocolStack: verify that TimeScheduler gets garbage collected on redeployment
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                 Key: JGRP-755
                 URL: http://jira.jboss.com/jira/browse/JGRP-755
             Project: JGroups
          Issue Type: Task
            Reporter: Bela Ban
         Assigned To: Bela Ban
             Fix For: 2.7
         Attachments: ThreadLeakTest.java

ProtocolStack hold a static ref to TimeScheduler, making the timer a JVM singleton. However, what happens if the channel is redeployed and thus reloaded in a different classloader ? The timer would be created again, because static refs are *per classloader*. 

The question is what happens to the old timers ? Are they GC'ed ?


[Brian] 

Following up from our chat yesterday re: static initialization of TimeScheduler.

My memory re cleanup *was* off. I was thinking of some stuff in java.util.Timer; doesn't really apply.

As you suspected, if you have a static ref to a TimeScheduler you'll leak the time scheduler and whatever classloader loaded JGroups (i.e. if a user deployed JGroups in a sar or something).

This isn't exact, but it shows the logical reference chain:

ProtocolStack.class.timeScheduler --> TimeScheduler.poolThreads[0] --> Thread.contextClassLoader --> ClassLoader.loadedClasses --> ProtocolStack.class.timeScheduler

That looks circular, but the system holds a ref to the live Thread in the middle and that keeps the whole thing from being gc'd.

Perhaps a way to deal with this is to maintain a static WeakReference<TimeScheduler> and then access it via a static method. Any ProtocolStack holds a strong ref to the scheduler, preventing it being gc'd, but once all channels are closed no ProtocolStack == available for gc.

FYI, I've attached a little main() class that experiments with something analogous to holding a static ref to a TimeScheduler. You can see that with the static strong ref in place, the thread and its TCCL don't get gc'd but once only a static WeakReference is held, things get cleaned up.


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